From my experience, upgrading to a good HD is very impressive in terms of perceived performance for an internet/word/excell use or things of the like. IE for a non game/video user.
My wife's comp is a bit old now, but will upgrade easilly to the best IDE drive around. No SATA in that comp.
Browsing around, it seems that the seagate ST3320620A could be a good candidate as a fast IDE drive, but I'm not sure it's the best option available. Its commercial name is:
SEAGATE BARRACUDA 7200.10 - 320 GO - 16 MO
that comp doesn't need large amounts of Gb, since its 80Gb drive is almost empty...
it just needs a very fast ide drive that can improve the feel to it.
any comments welcomed.